The invention concerns equipment for playing back data that can be read from the data-storage tracks on a recorded medium by means of an optical pick-up when a beam of light is positioned over the tracks by a tracking circuit, whereby the optical pick-up is shifted to the desired track by a hunting or skipping or track-skipping pulse during the track-hunting process and halted there by a braking pulse.
Equipment of this type--compact-disk players, opto-magnetic equipment for recording and playing back, and recording and playback equipment for DRAW disks or videodisc players for example--are provided with an optical pick-up consisting of a laser diode, several lenses, a prismatic beam divider, and a photodetector. The design and function of an optical pick-up are described in Electronic Components and Applications 6 (1984) 4, 209-15.
The beam of light emitted by the laser diode is focused by lenses onto the compact disk, which reflects it onto a photodetector. The data stored on the disk and the actual value for the focusing and tracking circuits are obtained from the photodetector output signal. The aforecited passage calls the actual value for the focusing circuit a focusing error and the actual value for the tracking circuit a radial tracking error.
The element that controls the focusing circuit is a coil with an objective lens that travels along its optical axis within the magnetic field. The focusing circuit moves the lens back and forth to keep the beam of light emitted by the laser diode constantly on the compact disk. The tracking circuit, which is often called the radial drive mechanism, moves the optical pick-up radially over the compact disk, positioning the beam of light on the spiral data tracks on the disk.
The radial drive mechanism in some equipment comprises what is called a coarse-drive mechanism and of what is called a fine-drive mechanism. The coarse-drive mechanism is for example in the form of a spindle that radially moves the overall optical pick-up consisting of the laser diode, the lenses, the prismatic beam divider, and the photodetector. The fine-drive mechanism can tilt the beam of light radially, at a prescribed slight angle for example, allowing the beam of light, due to the tilting alone, to travel a short distance along one radius of the compact disk.
To ensure unobjectionable playback of data, whether video and audio in a videodisk player or just audio in a compact-disk player, the beam of light must not only be precisely focused on the videodisk or compact disk but also precisely guided along the data-storage tracks on the disk.
European Patent 0 035 288 describes a playback device of the aforesaid type wherein the hunting on the part of the optical pick-up from one track to another is controlled by a hunting pulse, called a "jump pulse" in that text, and wherein the pick-up is halted in place above the desired track at the end of the movement by a braking pulse called a "compensate" pulse. Since, however, the braking pulse is always the same, it will not be optimal for every situation. The pick-up can for example halt ahead of or on the other side of the desired track, in which cases the access time to the desired data will be increased because subsequent corrections have to be made.
U.S. Pat. No. 4,495,608 discloses device for hunting tracks wherein the leap over one or more tracks is also initiated by a hunting pulse and terminated by a braking pulse. To make it possible to attain the desired track more precisely and rapidly, the duration of the braking pulse depends on the speed at which the pick-up travels over the tracks. The pulse is longer at higher speeds and shorter at lower speeds. It is, however, nevertheless possible with this device for the desired track not to be located immediately, so that subsequent corrections still have to be carried out. Critical situations can also occur in conjunction with the stability of this tracking circuit.